The present invention relates to semiconductor technology, in particular to a method of predicting chemical mechanical polishing (CMP) removal rates for CMP processes in a CMP process tool.
CMP process is commonly used to planarize the surfaces of insulating film and conductive film on a semiconductor substrate.
Polishing of materials such as silicon, silicon dioxide, tungsten, copper or aluminum is generally accomplished using a polishing pad in combination with suitable polishing slurry.
In addition, a wafer carrier is used in the polishing to hold the wafer such that the wafer faces the polishing pad dispersed with polishing slurry. The polishing pad is supported by a rotary or linear moving platen.
In general, a CMP process involves mechanically polishing a semiconductor wafer by holding and rotating the wafer against a polishing platen under different control parameters, such as a linear or rotational speed of a polishing pad, downward force by a wafer carrier, polishing time, flow rate and chemical conditions of CMP slurry. However, these parameters usually change with different CMP processes such that different removal rates are required. The obtained removal rate is essential for a CMP process and is applied to the CMP process to determine the necessary polishing time for the process.
Normally, a CMP process tool only performs one distinct CMP process such as inter-layer dielectric (ILD), inter-metal dielectric (IMD), or shallow trench isolation (STI) CMP process. The CMP process tool can also perform other CMP processes polishing materials with similar physical properties (e.g., dielectric material or metal). Thus, the CMP process performed by the CMP process tool can be transferred to reduce the WIP (wafer in process) of other certain CMP processes if necessary. A new removal rate is therefore needed when a CMP process performed by the CMP process tool is transferred.
FIG. 1 is a flowchart showing the process of transferring CMP processes in a CMP process tool according to the prior art. First, in step S1, at least one control wafer is polished under a first process with certain control parameters by a CMP process tool. The thickness of the control wafer is measured before and after the polishing by a measurement device and a thickness difference is obtained. Next, in step S2, a removal rate of the first process is obtained by calculating the process time and thickness difference of the control wafer. Next, step S3 determines whether the removal rate is acceptable by the first process. If so, the obtained removal rate is applied to the CMP process tool that performs the first process accordingly, as indicated in S4. If not, an operator (equipment engineer or process engineer) is informed and the process of the CMP process tool is halted and checked until the problem has been rectified, as indicated in step S5. After the engineer's inspection in step S5, steps S1˜S3 are repeated to obtain another removal rate for the first process to ensure that the problems have been rectified. Next, step S6 determines whether other incoming product wafers require polishing by the CMP process tool. If not, the CMP process is finished and the CMP process tool becomes idle, as indicated at step S7. Next, step S8 determines whether the incoming product wafers require the same process. If so, the CMP process tool continues to polish these wafers, as indicated in S9. If the incoming product wafers require different process with different control parameters, a new removal rate is needed to apply to the CMP process tool to determine a new process time and allows the process performed by the CMP process tool to be transferred. Thus, in step S10, at least one control wafer is polished under the control parameters of a new process by the same CMP process tool. The thickness of the control wafer is measured before and after polishing by a measurement device and a thickness difference is obtained. Next, in step S11, a removal rate is obtained by calculating the process time of the control parameters of the new process and the control wafer's thickness difference by the new process. Next, step S12 determines whether the new removal rate meets the process specification of the new process. If so, the newly obtained removal rate is applied to the CMP process tool, which starts the new process to polish incoming product wafers of the new CMP process, as indicated in S13. If not, an operator (equipment engineer or process engineer) is informed and the process of the CMP process tool is halted and checked until the problem has been rectified, as indicated in step S14. After the engineer's inspection in step S14, steps S10˜S12 are repeated to obtain another removal rate for the new process to ensure that the problems have been rectified and transfer of the CMP process is done.
Thus, it is necessary to perform another control wafer polishing sequence (monitor) as illustrated in steps S10˜S12 during transfer of the CMP process in a CMP process tool. The repeated monitor sequence requires considerable time to treat (preparing control wafers, measuring the thickness and particle condition before and after the CMP process) the control wafer, reducing real process time for polishing product wafers in a CMP process tool. In addition, the number of control wafers used in the second monitor sequence increases the process cost.
In U.S. Pat. No. 6,514,861, Yang et al. teach a semiconductor process for manufacturing at least one wafer that effectively controls process time by varying weighting factors. Prediction of the removal rate of chemical mechanical polishing that precisely controls polishing time is achieved by first providing a previously predicted process rate and a previously measured process rate by a process tool. Next, a presently predicted process rate is obtained by a first linear equation having a first variable weighting factor using the previously predicted and previously measured process rates as variables. Next, a process time is obtained according to the currently predicted process rate and a predetermined target to input to the process tool. Finally, the wafer is manufactured according to the process time by the process tool. U.S. Pat. No. 6,514,861, however, merely provides a method for precisely controlling the polishing time of a subsequent wafer of the same CMP process.
Accordingly, what is called for is a method of predicting a removal rate of subsequent CMP process and a method of transferring CMP processes in a CMP process tool in order to conserve process downtime, control wafers used, and resultant costs.